hosts: Add mtrcs0.p.r

*mtrcs0.pyrocufflink.red* is a Raspberry Pi CM4 on a Waveshare
CM4-IO-BASE-B carrier board with a NVMe SSD.  It runs a custom OS built
using Buildroot, and is not a member of the *pyrocufflink.blue* AD
domain.

*mtrcs0.p.r* hosts Victoria Metrics/`vmagent`, `vmalert`, AlertManager,
and Grafana.  I've created a unique group and playbook for it,
*metricspi*, to manage all these applications together.

group_vars/metricspi/blackbox.yml

@@ -0,0 +1,45 @@
+blackbox_modules:
+  icmp:
+    prober: icmp
+    timeout: 5s
+
+  http:
+    prober: http
+    timeout: 5s
+    http:
+      method: GET
+      headers:
+        Accept-Language: en-US
+        Accept-Charset: utf-8
+        Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
+
+  tcp:
+    prober: tcp
+    timeout: 5s
+
+  dns_recursive:
+    prober: dns
+    timeout: 5s
+    dns:
+      query_name: news.ycombinator.com
+      query_type: A
+
+  dns_pyrocufflink:
+    prober: dns
+    timeout: 2s
+    dns:
+      query_name: pyrocufflink.blue
+      query_type: SOA
+      validate_answer_rrs:
+        fail_if_not_matches_regexp:
+        - 'pyrocufflink\.blue\.\t\d+\tIN\tSOA\tdc\d+\.pyrocufflink\.blue.*'
+
+  smtp:
+    prober: tcp
+    timeout: 5s
+    tcp:
+      query_response:
+      - expect: "^220 ([^ ]+) ESMTP (.+)$"
+      - send: 'EHLO prober\r'
+      - expect: '^250[ -]SMTPUTF8'
+      - send: 'QUIT\r'